Rex the Boxer has degenerative myelopathy, a spinal cord disease that causes him to slowly become paralyzed starting from the rear.

To get around, the 10-year-old dog uses a wheelchair that supports his wobbly hind legs, but Smores the Guinea Pig has taken to hopping on and hitching a ride.

Their owner, Holly Walthers, told InsideEdition.com that everytime she prepares to let Rex out for a walk, she will tuck Smores into a custom basket attached to his wheels.

After the ride, Smores will jump out of the side cart all by herself, and let herself back into her cage.

"She's never once tried to jump out [during their walks]," Walthers said, amazed."She can get out if she wants, but she never does."

She adopted Smores a few months ago, after another one of her rescued Boxers died.

Walters said Rex was devastated after his former friend's death, and when she went to find a friend for Smores, she decided the two would be better together.

"[Rex] takes care of all my little dogs, and he lets one of my little dogs sit on his head and lick his face, so I knew he wasn't going to do anything," Walthers said. "I wasn't sure if Smores would like him so each day I put them together for a few minutes."